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Re: Monitor Problems

Warren Ernst wrote:

Mike,

Like I said, I am an amateur monitor repairman of at least 10 years.
I'm repaired at least 8 monitors successfully, out of maybe 12
attempts.

Regarding a charge: a monitor left unpowered for at least a day or so
should have next to no residual energy, but you always need to
discharge the tube before doing any work on ANYTHING on it.


Of course you should--and one learns to do this *almost* habitually. ;-)
But, once in a while, you will power up a monitor to test something,
then power it down, engrossed in the test results, and then brush
your hand agains the HV!  POP!  And another lesson is learned...

The point is that these events are practically never dangerous,
only "shocking"--unless you happened to have a hammer in your hand
and jerked toward the CRT envelope.  ;-)


A monitor that was powered on as recently has 45 minutes in the past
still makes a huge ZAP!!!! when you discharge it. From what I hear, the
charge is likely to travel from your right hand to your left hand,
right through your heart, if you are careless. That's where the real
hazard lies.


Yes, and that is why the "one hand in pocket" electricians rule is
a safe one.  But when did you last see a report of anyone killed or
injured by working on a TV or monitor?  What is possible and what
is likely are quite different.


To safely discharge a tube, you can use: a long screwdriver, a good
lenth of insulated heavy wire with aligator clips on each end, some
insulating gloves (optional, but I always use them.) Here's the
procedure:

1. Unplug the monitor and open the case. Don't touch anything
electronic inside. Optional: get a grounding wire and connect the metal
monitor chassis to an earth-ground. I don't bother with this anymore.

2. Clip the wire to a metal part of the chassis and to the base of the
screwdriver, near the handle.

3. Put on your gloves. Poke the end of the screwdriver under the
suction cup you see attached to the side of the tube (called the
annode). There is a metal clip at the center of the suction cup: make
sure the screwdriver tip touches the metal clip at the center. You may
hear a ZAP!!


Good procedure--essentially the same one I use.  But *do* make sure
that the screwdriver has an insulating plastic/rubber handle, just in
case.


4. If the monitor has been powered recently, I usually wait 5 minutes
and do it again, just to discharge any residual energy.


The "regeneration" of the charge is a result of remaining polarization
of the dielectric of the HV filter capacitor.  The relaxation time of
this dielectric depends on a number of variables, but just leaving the
HV terminal grounded is a good way to go.

Of course, if you do clip a ground wire to the HV, you will need to
tape the power plug to the ground wire, or otherwise do something to
prevent you from powering the monitor with the HV still grounded.


5. If you want, lift the edge of the suction cup to see the center
clip. Then upclip the annode.


However, both the HV lead (because of the HV filter capacitor) and
the anode terminal on the CRT (because of remanent polarization of
the glass bell of the CRT, which is also a capacitor) can "regenerate"
significant voltage after a short time--which is why grounding the
HV is the best protection against surprises.

I've worked on enough TVs and monitors (used to do TV repair) that
I usually don't worry much about it, but am just careful in the way
that being shocked in the past has trained me.  ;-)

Trying to train people to avoid shocks is good, but they *will* get
shocked occasionally.  Just as a welder will get burned from time to
time.  We're not perfect, that's life--learn from it!

While it's good to scare the "kids" away from playing with fire,
it's not good to keep responsible people in fear of learning a
new skill--even if there is some danger (isn't there always?).

Electricity is just electricity.  Shocks are just shocks.  Try
not to re-synchronize your heart with a powerful shock through
the chest.

The energy stored in a capacitor is 1/2*E^2*C, and typical output
capacitance of an HV supply is under 10^-9 Farads (.001 uF), so
at 20KV, that would be 20 Joules of energy (and that's how big
the POP is).  The usual external defibrillators deliver at least
50-100 Joules, and 150 Joules is considered average, so even if you
attached electrodes to your chest and popped yourself, you would be
well below what is usually "effective".  (A photoflash, on the other
hand, will store 2x - 10x more energy, and could be more dangerous.)


6. You're now good to go.

Hope this helps,
Warr


Michael J. Mahon wrote:



All things considered, I believe we overstate the danger when
we warn people about monitors.


-michael

New, faster SUDOKU v2.0 solver for Apple II's!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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